C# - Send Command To A Application In The System Tray To Launch A Form For Input

This is what I'm trying to accomplish: I have a Point of Sale application that integrates with a credit card processesor. I wan't to remove any user interfacing logic in regards to the credit card processor in the point of sale and add it to a application that sits in the background or system tray. I want to be able to send a command to the background app to popup a form for user input.

For example: User in the Point of Sale clicks a button "Credit Card" at this point it will send a command to the background app say "ProcessCC" which will pass a transactionid and an amount , this will open a form for the user to enter in the credit card information and submit the payment to the credit card proccessor.

Reason: I need to get a padss certification and don't want to submit the point of sale to do it. Rather send the simple background app that will control any credit card processing functionality such as encryption , storing etc. I know it can be done using sockets but was hopping for an alternative solution.